The present invention relates to an air conditioning system having a battery for dealing with an AC input voltage drop. More particularly, the invention relates to an air conditioning system provided with a converter for converting an AC input to a DC output, an inverter for transforming the DC output from the converter into an AC output which is supplied to a compressor under capacity control operation in accordance with the air conditioner load, and a battery for supplying DC output to the inverter via a switching device.
The operation of a conventional air conditioner system having a battery is as follows. The system, provided with a converter for converting power from a commercial power source into DC output and an inverter for transforming the converter output into AC output which is supplied to a motor for driving a compressor, is controlled to increase the input current as the power source voltage lowers. Therefore, there occurs the phenomenon where the power source voltage of a power transmission system at a high impedance will become further lowered. To deal with such a problem, a battery is provided. For example when an AC power source voltage becomes lower than a preset value, the battery is discharged to suppress the AC input current to a small value. A value set for starting a discharge of the battery and for stopping the discharge is not limited to such a single preset voltage value, but it is also possible to use a first set value of an input current for starting the discharge, and a second set value, smaller than the first set value, of an input current for stopping the discharge. Even with the first and second two input current values set as above, the air conditioning system can suppress the AC input current to a small value.
However, the conventional air conditioning system described above has the following problems associated therewith.
First, in the case of controlling with a set voltage value, a control circuit is provided which causes the battery to be connected to the input circuit of the inverter upon detection of the power source voltage becoming lower than the set value. The control circuit obtains its control power source voltages from an output circuit of the converter. Therefore, if a power failure occurs after the battery was connected because of a drop of the AC power source voltage at that time, the battery connection is maintained even after the power failure, since the control circuit obtains its necessary power from the battery. If the inverter continues to operate under such a condition, the battery is discharged in a short time and at the same time, is either damaged by an excessive discharge, or the life thereof is shortened.
In the case of controlling with two input current set values, the discharge current of the battery is controlled to have a constant value so that power which is otherwise to be supplied from the AC power source, is supplied from the battery. Therefore, the life of the battery is shortened and the running cost of the system becomes higher. This case will be clarified with reference to FIGS. 1 and 2.
The AC input current A is assumed to increase as shown by a solid line in FIG. 1 during the compressor capacity control operation. As the discharge starts when the current reaches a first set value Il, the battery discharge current B increases as indicated by a dotted line. At the same time, the AC input current A decreases to a value slightly larger than a second set value 12, the inverter current C taking the sum of both the currents. Thereafter, when the AC input current becomes the second set value 12 or less because of a reduction of the air conditioner load, the discharge is caused to stop. Then, the AC input current recovers a value slightly smaller than the first set value Il, and running only with the AC power source proceeds thereafter.
With the above control, a certain width or hysteresis width is necessary between the first and second set values. If power corresponding only to the hysteresis width is arranged to be supplied not from the battery but from the AC power source, and only insufficient power is arranged to be supplied from the battery as shown in FIG. 2, then the amount of consumption of the battery power which is relatively expensive, can be suppressed, and in addition, the number of chargings can be reduced. However, a conventional air conditioning system does not adopt such an arrangement, resulting in a shortened battery life and high running cost of the system.